1. Field of the Invention
The present invention relates to an image forming apparatus, which is applied to a printing section of a digital copying machine and a facsimile machine, and a digital printer, for forming an image on a recording medium by allowing developing particles to fly.
2. Description of the Related Art
There has existed an image forming apparatus for outputting an image signal as a visible image to a recording medium such as paper, which adopts a method generally called "xerography".
This image forming apparatus forms an electrostatic latent image by optical writing means on a developing substance having an electro-optics characteristics, i.e. photoreceptor, and the electrostatic latent image is developed by allowing toner which is developing particles to adhere. Thereafter the apparatus transfers the developed image to a recording medium such as a paper so as to form an image signal as a visible image on the recording medium.
In the above mentioned constitution, however, a developing substance having a special structure for forming an electrostatic latent image as well as writing means for the electrostatic latent image and charge eliminating means for erasing residual electric charge on the developing substance are required.
Furthermore, a constitution that the toner image formed on the photoreceptor is transferred to a recording medium is complex. For this reason, the structure of the apparatus becomes complex and there arises a problem that miniaturization of the apparatus is limited.
In this point of view, an image forming apparatus adopting a toner flying recording system, which forms an image by holding charged toner on a toner holding roller and allowing the toner to directly fly onto a recording medium by means of the Coulomb's force, is disclosed in Japanese Unexamined Patent Publication JP-A 1-503221(1989), Japanese Unexamined Patent Publication JP-A 7-186436(1995) and so on.
In the following context, an image forming apparatus adopting the conventional toner flying recording system will be explained.
FIG. 7 is a structural view of a conventional image forming apparatus, FIG. 8 is an enlarged view of a toner flying part of the conventional image forming apparatus, and FIG. 9 is a plan view of a control electrode in the conventional image forming apparatus.
In FIG. 7, a toner holder 102 is provided in a developing tank 101, and a toner supply roller 103 and a layer thickness restricting member 105 are pressed onto the toner holder 102. Further, a counter electrode 106 is provided opposite to the toner holder 102 with a control electrode 109 being interposed therebetween.
The control electrode 109 includes a plurality of electrodes (x-direction electrodes) arranged in a direction parallel to a longitudinal direction of the toner supply roller, a thin-film insulator 111 having a thickness of some tens of micrometer, and a plurality of electrodes 112 (y-direction electrodes) arranged in a direction intersecting to the x-direction electrodes 110, which are laminated in order, and at each intersection of the x-direction electrode 110 and the y-direction electrode 112 a toner passing hole 113 is formed.
An operation of the image forming apparatus having above described constitution will be explained below. In present case, it is assumed that toner 104 is negatively charged.
The toner 104 contained in the developing tank 101 is supplied to the toner holder 102 by means of the toner supply roller 103. At this time, the toner 104 is negatively charged due to a friction between the toner holder 102 and the toner supply roller 103 and supplied onto the toner holder 102.
The toner 104 adhered to the toner holder 102 is conveyed up to the layer thickness restricting member 105, and then charged again and restricted its layer thickness to a uniform thickness of between 10 .mu.m and 50 .mu.m by means of the layer thickness restricting member 105. After which, the toner 104 is conveyed to an opposing position of the control electrode 109.
The control electrode 109 connects with a control circuit 114 which generates a signal corresponding to image information and a driving circuit 115 which applies a voltage based on the signal. To the x-direction electrode 110 and y-direction electrode 112, Va volt is respectively applied when dot printing is carried out and Vb volt is applied when dot printing is not carried out.
To the toner holder 102 is applied Vs volt by an external power supply 116, and to the counter electrode 106 is applied Vt volt by an external power supply 117. The values of Va, Vb, Vs and Vt are predetermined so that flying of the toner may be controlled. That is, these values are determined so that flying of the toner may be controlled by electromagnetically changing the intensity of an electric field formed between the toner holder 102 and the counter electrode 106 by the potentials (Va, Vb) to be applied to the control electrode.
When Va volt is respectively applied to the x-direction electrode 110 and the y-direction electrode 112 during executing dot printing, the toner 104 conveyed in its negatively charged condition up to the opposing position of the control electrode 109 by the toner holder 102, receives an electric field higher than a toner flying start electric field, which causing the toner 104 to fly toward the toner passing hole 113.
The toner 104 having flown up to the toner passing hole 113 receives the force of an electric field along the direction of a recording medium 107 by the counter electrode 106 to which Vt volt is applied, thereby transferring onto the recording medium 107. During not carrying out the dot printing, since Vb volt is applied to either one or both of the x-direction electrode 110 and the y-direction electrode 112, the electric field does not reach the toner flying start electric field and therefore the toner 104 with negative charge would not fly toward the toner passing hole 113.
In order to represent dots as a continuous linear image, the toner passing holes 113 are arranged so as to form four toner passing hole 113 groups which are parallel to the longitudinal direction of the toner holder 102, in a condition that adjacent dots partially overlap to each other. And the image is formed by changing a control timing for each toner passing hole 113 group formed in parallel to the toner holder 102.
Finally, the recording medium 107 on which a visible image is formed is conveyed to a fixing roller 108 and the visible image is fixed on the recording medium 107, thereby obtaining a final image.
However, the conventional constitution is found to have a problem that normal images can not be stably formed for a long period because the toner will accumulate on a surface of the control electrode 109 or the toner will block the hole of the toner passing hole 113.
It is found that this problem tends to arise when there exist a lot of oppositely charged toner and weakly charged toner. Further, through observation of the flying state of the toner, it is found that the toner does not fly particle-by-particle but fly in clusters each composed of several to several tens of toner particles. Accordingly, the toner may break up during flight to fly to the directions different from the initial flying direction, and probably this fact would contribute the above mentioned problem.
For a solution of this problem, implementation was made to add in the developing tank an apparatus for eliminating the oppositely or weakly charged toner (in Japanese Unexamined Patent Publication JP-A 8-6383(1996), for example), but this solution has a problem of making the apparatus complex and increasing the cost.